They're often derided as rent-a-cops, the hall monitors of law enforcement, whose uniforms suggest professionalism and proficiency even if they don't always garner respect. In the Washington area, private security guards are legion, if largely unheralded, protecting banks and businesses, government buildings and museums, adding another layer of protection to the well-fortified capital.

The job can be boring, hour upon tedious hour of directing tourists to the nearest bathroom and asking people to remove the change from their pockets before passing through metal detectors. But the guards train for the worst.

And when the worst happened Wednesday at the U.S. Holocaust Memorial Museum, authorities said the Special Police Officers on duty were ready.

As they investigate the shooting, in which James W. von Brunn, an 88-year-old white supremacist from Annapolis, is charged with firing a rifle and killing one of the officers, authorities said yesterday that if it weren't for the quick response of the private guards on duty, more people could have been killed or wounded.

At a news conference yesterday, Mayor Adrian M. Fenty (D) said the officers' efforts "to bring this gunman down so quickly literally saved the lives of countless people. . . . This could have been much, much worse."

It started out as a normal day. Dozens of visitors, including school groups, were passing through the museum as the beginning of the summer tourist season began in earnest. Then, police said, a red Hyundai driven by von Brunn double parked on 14th Street outside the museum's entrance. When he got to the museum's door, Stephen T. Johns, who had been posted at the museum for six years, opened the door, police say.

Von Brunn then lifted his rifle and shot Johns in the chest at close range, officials said. Two officers immediately returned fire, hitting Brunn in the face, and he fell backward out the door, police said.

All of the officers were employees of Wackenhut Services, a large private security firm that provides protection for several government buildings, including the Federal Aviation Administration and the Nuclear Regulatory Commission. At the museum, the guards look just like police officers, with crisp uniforms, .38-caliber revolvers on their hips and silver badges with an image of a lion on a scale on their chests.

The Special Police Officer is different from a regular security guard, who "is a civilian who has no police authority," said D.C. Police Lt. Jon Shelton. SPOs, by contrast, are commissioned by the D.C. police chief and have "full police authority," including arrest power on the premises they are assigned to protect, he said.

To carry a firearm, SPOs must compete a 40-hour training session and go through an eight-hour recertification program every year to stay active, Shelton said. About 5,000 SPOs are licensed to work in the District.

Yesterday, Wackenhut declined requests to interview the officers who returned fire Wednesday. Officers at the museum said they were not permitted to comment.

In a statement, the company said it has provided security for the museum since 2002 and employs 70 officers there. William S. Parsons, the museum's chief of staff, said officers "train all year round, and all that training paid off in 50 seconds."

Many of the Wackenhut officers are former service members and have experience as police officers, said Assane Faye, the Washington district director of the Security, Police and Fire Professionals of America, the union that represents the Wackenhut guards. "There's more value than a rent-a-cop," he said. "They have high qualifications. . . . They put their lives on the line."

On a recent tour of the museum, Alexandria Sheriff Dana A. Lawhorne came away impressed with its security. "I noticed the guards seemed very attentive, alert, not sitting around," he said. "They were very strategically placed."

He said a security official there told him that officer training had been ramped up recently, including having guards run up stairs and fire their weapons "to get them to learn how to shoot with their heart racing and adrenaline pumping."